Delphi label and asm weirdness?

assembly, basm, delphi, delphi-7, x86

Solution

The practical advice is: do not use label keyword in asm code, use @@-prefixed labels:

function f(x: Cardinal): Cardinal; register;
asm
  not eax
  mov edx,eax
  shr edx, 1
  and eax, edx
  bsf ecx, eax
  jz  @@err
  mov eax, 1
  shl eax, cl
  mov edx, eax
  add edx, edx
  or  eax, edx
  ret
@@err:
  xor eax, eax
end;

Updated:

I have not found the bug report in Basm area. It looks like a bug, but I have used BASM for many years and never thought about using label keyword such a way. In fact I never used label keyword in Delphi at all. :)

Problem

I written an asm function in Delphi 7 but it transforms my code to something else: ``` function f(x: Cardinal): Cardinal; register; label err; asm not eax mov edx,eax shr edx, 1 and eax, edx bsf ecx, eax jz err mov eax, 1 shl eax, cl mov edx, eax add edx, edx or eax, edx ret err: xor eax, eax end; // compiled version f: push ebx // !!! not eax mov edx,eax shr edx, 1 and eax, edx bsf ecx, eax jz +$0e mov eax, 1 shl eax, cl mov edx, eax add edx, edx or eax, edx ret err: xor eax, eax mov eax, ebx // !!! pop ebx // !!! ret // the almost equivalent without asm function f(x: Cardinal): Cardinal; var c: Cardinal; begin x := not x; x := x and x shr 1; if x <> 0 then begin c := bsf(x); // bitscanforward x := 1 shl c; Result := x or (x shl 1) end else Result := 0; end; ``` Why does it generate `push ebx` and `pop ebx`? And why does it do `mov eax, ebx`? It seems that it generates the partial stack frame because of the `mov eax, ebx`. This simple test generates `mov eax, edx` but doesn't generate that stack frame: ``` function asmtest(x: Cardinal): Cardinal; register; label err; asm not eax and eax, 1 jz err ret err: xor eax, eax end; // compiled asmtest: not eax and eax, $01 jz +$01 ret xor eax, eax mov eax, edx // !!! ret ``` It seems that it has something to do with the `label err`. If I remove that I don't get the `mov eax, *` part. Why does this happen? Made a bug report on Quality Central.

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